Traveling-wave tube



May 23, 1961 c. K. BIRDsALL ErAL TRAVELING-WAVE TUBE Filed April 6.1955,

-NSM-N.

2,985,793 Patented May 23, 1961 ICC TRAVELING-WAVE TUBE Charles K.Birdsall, Los Angeles, and George R. Brewer,

Palos Verdes Estates, Calif., assignors to Hughes Aircraft Company,Culver City, Calif., a corporation of Delaware Filed Apr. 6, 195s, ser.No. 499,586

4 Claims. (c1. als-3.o

This invention relates to microwave tubes and more particularly toelectron stream amplifiers of the travelingwave type.

In a traveling-wave tube a helical conductor is generally employed topropagate an electromagnetic wave at a velocity substantially less thanthe velocity of light. An electron stream is then projected lengthwisethrough the center of the'helix whereby a mutual interaction of thestream and the wave causes the wave to grow or to be amplified.According to small signal theory an electromagnetic wave, when launchedalong the helix, actually divides into three forward waves. These threeforward waves comprise a growing wave, a decaying wave, and a wave of arelatively constant amplitude. As the electron stream enters the helixit begins to interact with the wave iields and becomes modulated. As thewave and stream progress the growing wave eventually dominates. Thetotal voltage of this wave acts on the electron stream; this is thevoltage representative of the toal A C. energy propagated by both thecircuit or helix and the stream. This wave energy can also be dividedinto two portio'ns comprising that present in the circuit and that inthe stream. These portions are represented by the two components of thetotal voltage, called the circuit voltage and the stream voltage. At theoutput end of he helix only he energy represented by the circuit voltagemay be coupled to an external load. The power output and efficiency of atraveling-wave tube is therefore limited by the fact that the circuitvoltage may be considerably less than the total voltage at the output ofa helix. The elecron stream velocity is uniform along a single helix andis of course determined by the direct-current voltage of the helix. Thestream velocity for maximum gain, however, is usually different thanthat required fo'r producing a maximum ratio of circuit voltage to totalvoltage.

It is therefore an object of this invention to provide means forincreasing the 4power output of a travelingwave tube.

Brewer. The two helix segments, in the instant case, may thus bemaintained at different direct-current potentials. The principal helix,which is positioned near the input end of the tube, is, in fact,maintained at a selected voltage to cause the electron stream to ow nearits maximum gain velocity, i.e. that electron stream velocity whichproduces maximum gain. However, the smaller or output helix ismaintained at a voltage to cause the stream to flow at a velocity whichwill produce a maximum transfer or maximum output coupling e'iciency ofelectromagnetic energy from the stream to the circuit.

The novel features lwhich are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawing in which several embodiments of the invention areillustrated by way of example. It is to be expressly understood,however, that the drawing is for the purpose of illustration anddescription only, and is not intended as a definition of the limits ofthe invention.

Fig. 1 is a sectional view of a traveling-wave tube amplier illustratingone embodiment of the present inventio'n; and

Fig. 2 is a broken sectional view of an alternative embodiment of theinvention.

Referring now to the drawing, there is shown in Fig. l an embodiment ofthe present invention which comprises a `traveling-wave tube 10including input and output matching cavities 12 and 14 with input andoutput co- Y axial cables 16 and 18, respectively connected to the It isanother object of the invention to provide means for increasing theefficiency of a traveling-wave tube.

It is a further object of the invention to provide means whereby `Waveenergy may be more effectively extracted from the electron stream of atraveling-wave tube at the output end of the tube helix.

In accordance 'with the present invention the helix of a traveling-wavetube is severed near the output end and means are provided to couple theelectromagnetic wave energy from one of the helical segments to theother. A similar structure is employed at the input end of the tube in again control system which is described and claimed in copendingapplication Serial No. 470,205, entitled Traveling-Wave Tube GainControl, filed November 22, 1954, by George R. Brewer, now U. S. Patent2,869,-

022, issued January 13, 1959. A similar high gain strucseparatecavities. An envelope 20, which provides the evacuated chamber oftraveling-wave tube 10, consists of a long cylindrical structure whichhas an enlarged portion at the left extremity, as viewed in Fig. l.Within the enlarged portion of the envelope there is located an electrongun 22 for producing a stream of electrons which is directed along apredetermined path that lies on the longitudinal axis of envelope 20.

A solenoid 24 is positioned coaxially about the elongated portion ofenvelope 20. An appropriate direct-current is maintained in solenoid 24by means of a potential source, such as a battery 26, so as to produce amagnetic tield of the order of 1000 gauss, whereby the stream isconfined over the complete length of the elongated portion of the tube10.

lElectron gun Z2 comprises a filament 28, a cathode 30, a focusingelectrode 32, and an accelerating ano'de 34. Filament 28, which issupplied with current by a potential source 36, is employed to heatcathode 30, the cathode being connected to the positive side of filament28. Focusing electrode 32, sometimes called a Pierce electrode, has a-frusto-conical configuration with an internal surface of revolutionwhich may be disposed 671/2 mechanical degrees from its axis ofsymmetry. Electrode 32 is maintained at a potential somewhat negativewith respect to cathode '30 by a connection to the negative terminal ofsource 36. Ano'de 34 is maintained at ground by a suitable connection 35thereto. `Cathode 30 is maintained at about 800 volts negative withrespect to ground by a source 40, the negative terminal of which isconnected to source 36. A contact arm 38 on source 40 is connected toano'de 34 and hence to ground.

Disposed concentrically within envelope 20 about the electron streampath in the direction of ow are a matching ferrule 42 connected over alead 44 to a principal helix 46. A secondary or output helix 48 isconnected over a lead y49 to a matching ferrule 50.

Principal helix 46, by means of a connection to the positive terminal ofsource 40 through a ferrule 42, is maintained at a potential to producemaximum gain.

Output helix 48 is maintained at ground potential by a connectionthrough ferrule 50. However, helix 48 may be maintained at any otherpotential which produces maximum eciency.

Disposed about envelope and contiguous thereto is coupling helix 68which is disposed coextcnsive with the separation in space of principaland output helices 46 and 48. All of the helices 46, 48 and 68 areconstituted of a stiff metal such as tungsten or molybdenum since it isrequired that they retain their shape, especially with respect to theirpitch -to diameter ratios. In this embodiment of the invention it isintended that the diameters and pitch angles of principal and outputhelices 46 and 4S be the same and that coupling helix 68 have a pitchangle which is the negative of that of principal and output helices 46and 48. It is unnecessary to maintain coupling helix 68 at any specificreference potential.

The stream electrons are intercepted by a collector electrode 52 at theend of envelope 20 opposite electron gun 22. Collector 52 is sealed toenvelope 28 so as to have a large surface external to the evacuatedchamber for heat dissipation purposes and may include fins asillustrated to aid in conducting away the heat that is dissipated by thestream electrons when collected. Accordingly, collector 52 is preferablyfabricated from a metal having good electrical and heat conductingproperties such as, for example, copper or silver. A potential of theorder of 200 volts positive with respect to that applied to the outputhelix 48 is applied to collector 52 in order to prevent secondaryelectrons from reaching the output helix 48 or ferrule S0. Thispotential is applied by means of a connection from collector 52 to thepositive terminal of a source 54, the negative terminal of which isreferenced to ground.

As previously mentioned principal helix 46 is connected to ferrule 42 bylead 44 and output helix 48 to ferrule 50 by lead 49. Leads 44 and 49are located parallel to the electric fields excited within matchingcavities 12 and 14. Matching cavity 12 has the configuration of arectangular toroid with a concentric collar 56 disposed about matchingferrule 42 and spaced from envelope 20 and a slot opening adjacent tolead 44. An opening 58 in the end plate of cavity 12 facing the left endof principal helix 46 allows the full length of lead 44 to be energizedand, in addition, decreases the tendency of the electric eld producedbythe potential in the cavity from disturbing the flow of the electronsin the streams. Cavity 14 is similarly shaped, having a correspondingconcentric collar 60 arranged about matching ferrule y5t) and spacedfrom the envelope and an opening 62 facing the right end of output helix48.

The two respective center conductors v6-4 and 66 of coaxial cables 16and 18 extend through the apertures in the end plates of cavities 12 and14 to be in electrical contact with collars 56 and 60, respectively,while the outer conductors of the cables 16 and 18 are bonded to theperiphery of the apertures. Cavities 12 and 14 are fabricated with aninner surface composed of a highly conductive material and are broadlyresonant so as not to limit the frequency of operation. The congurationshown for cavities 12 and 14 in the drawing may provide, Afor example,suitable matching from helix 46 to coaxial cables 16 and 18 over a 2:1range of frequencies extending from 2000 to 4080 megacycles.

In the operation of the tube 10 an input signal is applied to the centerconductor 64 of coaxial cable 16. An electric eld is thereby excitedwithin resonant cavity 12 which energizes lead 44 to launch atraveling-wave along principal helix 46. The electron stream supplied bygun 2.?. is then directed through principal helix 46 to interact withthe wave. Principal helix 46 is maintained at or near the direct-currentpotential necessary for producing maximum gain. Near the output end ofthe tube it) the wave energy is coupled from principal helix 46 tooutput helix '48 by coupling helix 68, output helix 48 being maintainedat or near a direct-current potential for producing a maximum circuitvoltage at the output end of output helix 48. Output helix 48 thusefciently demodulates the modulation of the stream electrons, theelectrons being subsequently intercepted by collector 52. The circuitvoltage on output helix 48 being a maximum, a maximum transfer of waveenergy is effected as the wave flowing along lead 49 excites analternating electric field within cavity 14. The energy is thenwithdrawn from cavity 14 when the output signal is launched on thecenter conductor 66 of output coaxial cable 18.

An additional explanation of the physical mechanism of operation of thepresent invention may help to make clear its novel features. Asmentioned before, the interaction between the electron stream and thewaves propagating on the circuit results in electron bunching so thatA.C. energy can be associated with the fields of the space charge andwith the elds on the circuit. Only the wave on the circuit is coupledout to a useful load. It is there fore desirable that as much aspossible of the space charge energy be converted to circuit energy priorto the output coupling for maximum power output and efciency. Asingle-circuit tube is ordinarily adjusted for maximum Ipower output sothat this voltage must be a compromise between that required for maximumgain and that for maximum conversion of space charge-to-circuit energy.This compromise will not in general result in optimum performance. Thepresent invention allows adjustment of the electron velocity to achieveeach `of these effects separately; namely, adjustment of the principalhelix potential for maximum gain, and adjustment of the output helixpotential for maximum power output. By the use of the structuredisclosed in the present invention, more nearly optimum performance ofthe traveling-wave can be obtained.

Another method for producing a direct-current potential separation andan alternating-current coupling for the principal and output helicesemployed in the present invention is shown in Fig. 2. This brokensection of the output end of a traveling-wave tube exhibits an evacuatedenvelope enclosing a principal helix 122 with an output helix 124. Inorder that an intermediate helix be eliminated, output helix 124 hasbeen contra-wound about a portion of principal helix 122, i.e. outputhelix 124 has a negative pitch angle, i.e. an angle equal in magnitudebut opposite in sign to the pitch angle of principal helix 122.Furthermore, output helix 124 has a larger diameter than that ofprincipal helix 122.

A helix is not, of course, the only type of slow-wave structure withwhich the present invention may be employed. Numerous other types, suchas a disc-loaded waveguide, rare illustrated in Traveling-Wave Tubes byJ. R. Pierce, D. Van Nostrand and Co., Inc., New York, 1950. Anothertype-is also shown in copending application Serial No. 450,987, entitledHigh Power Microwave Tube, tiled August 18, 1954, by C. K. Birdsall, nowU.S. Patent 2,957,103, issued October 18, 1960. In other slow-wavestructures a principal and an output section would thus be electricallyinsulated. The present invention would also include means formaintaining the sections at appropriate direct-current potentials andmeans for coupling electromagnetic energy from one section to the other.

The pitch and diameter of principal helix 46 and output helix 48 aredetermined by the operating frequency band desired and the powerrequirements of the tube 10. As stated previously, principal helix 46 ismaintained at a potential to produce maximum gain of the growing wave.All that is left in the design of the tube 10 is then the respectiveoptimum lengths of principal helix 46 and of output helix 48 and thedetermination of the potential of output helix 48 at which maximumeiciency will occur. By employing well-known wave propagation lequationsin a traveling-wave tube, thegain of a travelingwave tube may be plottedversus the direct-current pctential of output helix 48 for differentlengths. A curve of maximum gain then may be plotted versus tube length.Where maximum gain itself reaches a maximum value, an optimum length foroutput helix 48 may be found. By plotting gain versus the direct-currentvoltage of output helix 48 `for the optimum length of output helix 48,an optimum voltage for output helix 48 may be determined. An overalltube length will generally be assumed or will be determined by the tubepower requirements. Thus the length of principal helix 46 may dependonly on the optimum length of output helix 48.

In the design of the tube of the present invention, curve calculationsand plotting is suggested as a preferred method of determining anoptimum design because of the complexity of traveling-wave tube wavepropagation equations. A direct analytical approach is feasible, butgenerally impractical.

What is claimed is:

1. A microwave tube comprising electron gun means for projecting astream of electrons along the axis of said tube, a principal slow-wavestructure helix for propagating traveling waves to modulate saidelectron stream, an output slow-wave structure helix aligned coaxiallywith and disposed adjacently to but direct-current insulated from saidprincipal slow-wave structure helix and being exposed to the modulatedelectron stream for propagating electromagnetic waves in energy exchangerelationship therewith, and helical means disposed intermediate andcoaxially with said helices for coupling said electromagnetic energyfrom said principal slow-wave structure to said output slow-wavestructure helix Without appreciable phase delay therebetween, saidprincipal slow-wave structure helix being of a predetermined length andbeing maintained at a direct-current potential to produce maximumamplification of said electromagnetic energy, and said output slow-wavestructure helix being of a substantially shorter predetermined lengthand being maintained at a direct-current potential to produce a maximumratio of circuit voltage to total voltage associated with saidelectromagnetic energy.

2. A traveling wave type amplifier tube comprising means for projectingan electron stream along the axis of said tube, a principal conductivehelix disposed concentrically about said electron stream for propagatingan electromagnetic Wave at a predetermined velocity and for modulatingsaid electron stream and being maintained at a direct-current potentialto produce maximum amplification of said electromagnetic wave, an outputhelix disposed in electromagnetic communication and energy exchangerelationship with said stream and aligned coaxially with and having alength substantially less than that of said principal helix and disposedadjacently thereto but direct-current insulated therefrom, helix meansdisposed intermediate and coaxially with said helices for couplingelectromagnetic wave energy substantially without phase delay from saidprincipal helix to said output helix so that said output helixsubsequently propagates said electromagnetic wave, and means formaintainingr said output helix at a direct-current potential to producemaximum output coupling eciency from said stream to said output helix.

3. A traveling Wave amplifier tube comprising an evacuated envelope andan electron gun disposed at one end of said envelope for producing anelectron stream, means for directing said stream along a predeterminedpath including the longitudinal axis of said envelope, a collectorelectrode disposed at the opposite end of said envelope for interceptingsaid stream electrons, a principal helix disposed about said pathadjacent said electron gun for propagating an electromagnetic wave at a.predetermined velocity n energy exchange relationship with saidelectron stream for modulating said stream, said predetermined velocitybeing small in comparison to the velocity of light, an output helixdisposed sequentially and coaxially along said path and exposed to themodulated stream for propagating said wave at a velocity greater thansaid predetermined velocity, said output helix being substantiallyshorter than said principal helix, helical means coaxial with saidhelices for coupling said electromagnetic wave without phase delay fromsaid principal helix to said output helix, means for maintaining saidprincipal helix at a direct-current potential to produce maximumamplification of said wave and means for maintaining said output helixat a direct-current potential to produce a maximum ratio of circuitvoltage to total voltage.

4. A traveling wave amplier tube comprising an evacuated enevlope, anelectron gun disposed at one end of said enevlope for producing anelectron stream, means for directing said stream along a predeterminedpath including the longitudinal axis of said envelope, a principal helixof -predetermined length, pitch and diameter for modulating said streamto provide maximum amplification of traveling waves propagatingtherealong at a first predetermined velocity and being disposedconcentrically about said axis adjacent said electron gun, an outputhelix exposed to the modulated electron stream and having apredetermined length, pitch and diameter to provide maximum couplingfrom said stream and being disposed sequentially along said pathadjacently to said principal helix but direct-current insulatedtherefrom for propagating said wave at a velocity greater than saidiirst velocity, said output helix being substantially shorter than saidprincipal helix, external contrawound helix means for coupling saidelectromagnetic wave from` said principal helix to said output helixwithout appreciable phase delay, means for maintaining said principalhelix at a direct-current potential to produce maximum amplication ofsaid Wave and means for maintaining said output helix at adirect-current potential to produce a maximum ratio of circuit voltageto total voltage to thereby maximize the output coupling efficiency ofsaid tube.

References Cited in the tile of this patent UNITED STATES PATENTS2,541,843 Tiley Feb. 13, 1951 2,588,831 Hansell Mar. 11, 1952 2,588,832Hansell Mar. l1, 1952 2,623,193 Bruck Dec. 23, 1952 2,660,689 Touratonet al. Nov. 24, 1953 2,733,305 Diemer Ian. 31, 1956 2,802,136 LindenbladAug. 6, 1957 2,804,511 Kornpfner Aug. 27, 1957 2,849,651 Robertson Aug.26, 1958 2,869,022 Brewer Ian. 13, 1959 2,885,593 Cook May 5, 19592,894,168 Wing et al. July 7, 1959 2,905,858 Cutler Sept. 22, 19592,928,979 Kompfner Mar. 15, 1960 FOREIGN PATENTS 969,886 France May 3l,1950 1,071,367 France Mar. 3, 1954 OTHER REFERENCES Article by P. D.Lacy, pages 132-135, Electronics for November 1954.

